Audio storage and reproduction in recent years has been dominated by the digital audio medium which typically utilizes a laser scanned Compact Disc (CD) player. The CD player utilizes a disc upon which digital audio information is encoded in a predetermined format, which format can then be read by a laser. The digital data is organized in spiralling tracks on the surface of the disc, somewhat similar to a conventional phonograph record. This digital data is organized in sequentially generated sample frames, each sample frame containing both audio information and sample position information. The CD player is therefore operable to provide general information as to relative location on the disc of each sample frame such that it is possible to electronically relocate the scanning device to a particular location on the disc.
All data on a CD is formatted by frame; by definition a frame is the smallest complete section of recognizable data on a disc. Each frame contains a 24 bit synchronization word plus three merging bits, eight bits of unmodulated subcode, six sixteen-bit audio words, thirty two bits of parity, six sixteen-bit audio words and thirty two bits of parity. These unmodulated bits are eight-to-fourteen modulated (EFM), interleaved, and combined with merging bits and the 27 bit synchronization word for a total of 588 bits per frame. Since each 588 bit frame contains 12 sixteen bits per audio samples, the result is 49 channel bits per audio sample. The frames are disposed on spiralling tracks which are adjacent to each other and separated by approximately 1.6 micrometers. The data is encoded by disposing selectively spaced and sized pits in the surface of the disc, which pits are then scanned with a laser while the disc is rotating. Unlike an analog recording on a typical phonograph record, the data is read from a given track at a rate corresponding to the speed with which the track passes the scanning element. If the disc were to rotate at a given speed, this would mean that data would come off at a faster rate proximate to the peripheral edge of the disc as compared to the center portion of the disc. This data is typically placed into a first-in first-out (FIFO) buffer and the speed of the disc controlled to ensure that the buffer does not overflow or underflow. Thereafter, a sampling clock is provided to read the information from the FIFO at a predetermined output rate. This is typically controlled by a word clock.
When recording data for multiple channels such as stereo, quadraphonic, etc. applications, it is necessary that the phase of the different channels be synchronized to ensure that the audio is properly reproduced. It can be appreciated that as long as data is recorded onto the same CD, this can be easily achieved since there is one word clock and a common speed for all tracks on a given disc. Therefore, a single disc facilitates multiple channel recording. However, as the number of channels increases, the amount of data per channel that can be recorded decreases, since the disc has a standard and finite size.
In order to provide long playing audio programs with multiple channels, it is necessary to utilize more than one CD player, thus requiring multiple discs. Of course, this now requires multiple word clocks and multiple rotating mechanisms. The word clocks for the multiple players can be synchronized since some CD players provide an external input for the word clock, which word clock can therefore be common to all players. However, there is no common link between each of the CD players to ensure that the samples are properly aligned.
Sample alignment typically requires that the audio samples, when compared side-by-side between the channels, be output in the same sequence from the beginning of the audio program. For example, if there were two thousand samples in an audio program, it is important that the one thousandth sample in the first channel be output at substantially the same time as the one thousandth sample in the nth channel is output. Any misalignment in the samples will be noticed. Although subcode that is embedded in each of the sample frames contains information about its general location in a particular program, synchronizing to this information can result in a substantial amount or jitter. This is due to the fact that the subcode data is output from the digital data stream on a separate path from that on which the audio data is flowing. The processing for both paths can differ by as much as .+-.3 milliseconds. This would be unacceptable for high quality audio reproduction with multiple channels.
In view of the above disadvantages and limitations of CD systems, it is desirable to provide a system that will allow multiple CD players to be synchronized on a sample by sample basis.